Method of fabricating a capacitive environment sensor

ABSTRACT

A method for fabrication of capacitive environment sensors is provided in which the sensor elements are integrated in a CMOS structure with electronics through the use of complementary metal oxide semiconductor (CMOS) fabrication methods. Also provided are environment sensors fabricated, for example, by the method, and a measurement system using the environment sensors fabricated by the method. The described method includes etching away one of the metal layers in a CMOS chip to create a cavity. This cavity is then filled with an environment-sensitive dielectric material to form a sensing capacitor between plates formed by the metal adhesion layers or an array of contacts from other metal layers of the CMOS structure. This approach provides improved sensing capabilities in a system that is easily manufactured.

STATEMENT REGARDING FEDERALLY-SPONSORED RESEARCH AND DEVELOPMENT

This invention was made with government support under Grant Nos.NIOSH/CDC 200-2002-00528 and AFOSR FA9550-07-1-0245. The government hascertain rights in this invention.

This invention relates generally to the field of chemical sensing. Inparticular, it relates capacitive chemical sensors integrated, forexample, into a Complementary Metal Oxide Semiconductor (CMOS) structureor like structure.

Capacitive chemical sensors have traditionally been made by creating abottom electrode, depositing a sensitive material, and then patterning aset of top electrodes (also referred to herein as conductors). A diagramof this structure 10 is shown in FIG. 1 a, showing silicon 12 andsilicon dioxide 14 substrate layers, electrodes 16 and 17 andchemical-sensitive polymer layer 18. In reference to the Figures,electrodes that are numbered differently refer to opposite conductors (+versus −, as the case may be) in the capacitor structure. The silicondioxide layer serves as a dielectric, electrically insulating, layerthat is not sensitive to exposure to chemicals. Many other insulatingmaterials can serve this purpose. The silicon layer serves as amechanical substrate. Many other materials can also serve this purpose.This structure has high sensitivity, since all the electric field linesmust pass through the sensitive material. Using polyimide polymer,sensitivities reported when this method is applied to humidity sensingare approximately 0.2% change in capacitance for every 1% change inrelative humidity. However, this structure is difficult to integratewith testing electronics; placing a sensitive layer between two metallayers requires significant processing beyond conventional CMOS, and hasnot been successfully demonstrated.

As a result, other capacitive sensors have used an alternative approach,coating interdigitated metal electrodes with a sensitive film. Thissensor 20 is shown in FIG. 1 b, which depicts silicon 22 and silicondioxide 24 substrate layers, electrodes 26 and 27 and chemical-sensitivepolymer 28. This simplifies processing, by eliminating the necessity ofhaving metal above and below the sensitive layer, but at a cost ofcreating a large, parallel capacitance through the substrate under theelectrodes. A technique developed by Seacoast (United States PatentPublication No. 2006/0237310) raises the electrodes on a short verticalpost, but vertical posts are also difficult to integrate with CMOSprocessing.

A variant of this approach, developed by ETH Zurich (European PatentPublication EP 1 607 739 A1), leaves the oxide in the CMOS in place,coating the top surface of a foundry CMOS chip; this simplifiesfabrication, but further reduces the sensitivity since the most directelectric field lines pass through the oxide. A simplified diagram ofthis approach is shown in FIG. 1 c, depicting a sensor 30, and showingsilicon 32 and silicon dioxide 34 substrate layers, along withelectrodes 36 and 37 and chemical-sensitive polymer 38. The sensingcapacitance of the ETH Zurich device with polyurethane as the sensitivepolymer is 1.4 pF in parallel with a substrate capacitance of 6.4 pF.Since 18% of the total capacitance is affected by the analyte, thesensitivity is at most 18% of that of a parallel plate sensor (such asin FIG. 1 a), or about 0.04% change in capacitance per percent relativehumidity.

Another technique that has been used is to remove the underlyingsubstrate, leaving the electrodes on a thin dielectric membrane (UnitedStates Patent Publication No. 2003/0002238 A1); a diagram is shown inFIG. 1 d. In FIG. 1 d, sensor 40 is shown, along with silicon 42,silicon dioxide 44, electrode 46 and 47 and chemical-sensitive polymer48 structures. This has the effect of removing the parasitic capacitancebetween the electrodes and the substrate, but there will still be aparallel capacitance through the non-sensitive dielectric 44 that willdegrade the sensitivity. Accordingly, there is a need for improvedmethods, apparatuses, and systems for capacitance-based gas chemicalsensing which reduces parasitic capacitance, increasing miniaturization,while being manufacturable through low cost methods.

SUMMARY

A method of fabricating a chemical sensor utilizing Complementary MetalOxide Semiconductor (CMOS) fabrication techniques to produce anintegrated chemical capacitive sensor into a CMOS structure isdescribed. The method comprises selective etching of the dielectric ofthe CMOS (“CMOS dielectric”) to expose a metal layer within the CMOSelectrically connected in series between two other metal layers, etchingthe metal layer to provide a cavity, and filling the cavity with anenvironment-sensitive dielectric material. Alternately, the metal layermay comprise a core metal layer disposed between two metal adhesionlayers and etching of the metal layer can selectively remove only thecore layer or can remove both the core layer and the two metal adhesionlayers.

A chemical sensor integrated into a CMOS structure is also described.The chemical sensor comprises a CMOS structure having at least two metallayers and a CMOS dielectric; and at least one environment-sensitivedielectric material layer connected in series between two of the metallayers to produce a capacitor structure. The CMOS structure compriseschannels in the CMOS dielectric extending from a surface of the CMOSstructure to the environment-sensitive dielectric material layer suchthat at least a portion of the environment-sensitive dielectric materiallayer is exposed to the atmosphere or environment. The channels may ormay not comprise the environment-sensitive dielectric material.Alternatively, the environment-sensitive dielectric material layer isdisposed between two metal adhesion layers, each of which isindependently electrically connected to an adjacent metal layer.

A system for environment sensing comprising one or more such environmentsensors integrated into a CMOS structure integrated into circuitry forcapacitance measurement is also described. In one embodiment, at least aportion of the circuitry for capacitance measurement is integrated intothe CMOS structure with the sensors. In one non-limiting embodiment ofthis system, the circuitry of the CMOS into which the one or morechemical sensors are integrated is a charge-based capacitancemeasurement (CBCM) circuit.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 a shows a schematic of a prior art capacitive chemical sensorwith a sensitized layer on top of the electrodes;

FIG. 1 b shows a schematic of a prior art capacitive chemical sensorwith a sensitized layer between lateral electrodes;

FIG. 1 c shows a schematic of a prior art capacitive chemical sensorwith a sensitized layer between released lateral electrodes;

FIG. 1 d shows a schematic of a prior art capacitive chemical sensorwith a sensitized layer as a layer between parallel plate electrodes;

FIG. 2 a is a cross sectional diagram showing one option forimplementing a vertical parallel-plate chemical sensor into a CMOS bysubstituting polymer for a metal layer;

FIG. 2 b is a cross sectional diagram showing one embodiment of avertical parallel-plate sensor integrated into a CMOS according to thedescribed method where polymer is substituted for a metal core layerdisposed between metal adhesion layers;

FIG. 3 is a cross section of a typical CMOS useful in the describedmethod, apparatus, and system described herein.

FIGS. 4 a-4 d show diagrammatically one embodiment of the inventivemethod for fabricating a chemical sensor integrated into a CMOS;

FIGS. 5 a and 5 b show cross sections of one embodiment of a chemicalsensor integrated into a CMOS;

FIG. 6 a-6 c show diagrammatically another embodiment of the inventivemethod for fabricating a chemical sensor integrated into a CMOS;

FIG. 7 illustrates one embodiment of a computer system for use inimplementing the sensor described herein;

FIGS. 8 a and 8 b are SEM images of one embodiment of a CMOS with anintegrated chemical sensor (a) before and (b) after inkjetting ofpolymer with an inset SEM image of filled release holes;

FIGS. 9 a-9 c are a (a) CBCM circuit, (b) a timing diagram for a firstmeasurement and (c) a timing diagram for second measurement;

FIG. 10. shows a schematic of the humidity sensor test setup used totest the exemplary chemical sensor;

FIG. 11. is a humidity sensor response curve from testing done on theexemplary chemical sensor;

FIG. 12. is a graph showing a simulation of humidity concentrationbetween plates in response to stepped humidity pulse;

FIGS. 13 a and 13 b are graphs showing sensor response to humiditypulses for the exemplary chemical sensor;

FIG. 14 is a graph showing the Allan variance of the exemplary chemicalsensor integrated into a CMOS; and

FIG. 15 is data on the temperature variance of the exemplary chemicalsensor integrated into a CMOS.

DETAILED DESCRIPTION

The use of numerical values in the various ranges specified in thisapplication, unless expressly indicated otherwise, are stated asapproximations as though the minimum and maximum values within thestated ranges are both preceded by the word “about”. In this manner,slight variations above and below the stated ranges can be used toachieve substantially the same results as values within the ranges.Also, unless indicated otherwise, the disclosure of these ranges isintended as a continuous range including every value between the minimumand maximum values. For definitions provided herein, those definitionsrefer to word forms, cognates and grammatical variants of those words orphrases.

A method for producing an environment-sensitive capacitor (e.g., aparallel-plate capacitor) which is configured into a Complementary MetalOxide Semiconductor (CMOS) structure. One or more sensor elements areintegrated into the CMOS structure through the use of CMOS fabricationmethods. Reference is made to the use of the chemical sensor as ahumidity sensor, but those skilled in the art will recognize that thechemical sensor represents a general motif that can be applied to manyenvironment sensing applications.

The method, device and systems described herein involve the introductionof an environment-sensitive capacitor into an electronic circuit of aCMOS structure, which is configured to sense a specific analyte. Thecapacitor uses an environment-sensitive material as a dielectric.According to one embodiment, as the analyte is absorbed into, orotherwise affects the dielectric material, the permittivity of thedielectric material is altered, which raises or lowers the capacitanceof the capacitor. By monitoring this change in capacitance, the analytemay be identified or quantified.

In an alternate embodiment, exposure of the environment-sensitivematerial to the analyte may cause the environment-sensitive material toexpand pushing the electrodes of the capacitor apart and changing thecapacitance. Again, by monitoring this change in capacitance, theanalyte may be identified or quantified.

As used herein, an analyte is any environment parameter that can bemeasured, including the amount of a particular chemical compound, suchas water (humidity), CO, CO₂, volatile organic compounds (VOCs), andother gases, etc., and a physical parameter, such as temperature. Anenvironment-sensitive dielectric material is not necessarily atraditional dielectric as is used in capacitors, but, as describedherein, is a material that changes its permittivity, or otherwisechanges the capacitance of the capacitor structure in response toexposure to an analyte.

The structures are described herein as CMOS structures. This is inreference, not to the electronic capabilities of the structure, but withregard to its physical structure and the ability to fabricate theminiaturized analog structures into the CMOS structure, such thatadditional electronic constituents can be fabricated into the same CMOSstructure as the capacitive sensor, including one or more additionalcapacitive sensors, permitting substantial miniaturization as comparedto prior structures which required tie-ins to PC board-mountedelectronics. Nevertheless, some variation in the metals andsemiconductor materials used in the CMOS device are contemplated, suchas the substitution of Cu for Al, the substitution of Ta for Ti/Wadhesion layers, or the substitution of SiO₂ with, e.g., a low-kdielectric (a low-κ dielectric, having a dielectric constant, x, lowerthan that of SiO₂).

In order to produce an environment-sensitive capacitor in a CMOS stack,conductive layers are provided on either side of a cavity. One suchstructure 50 is depicted schematically in FIG. 2 a. Although the layersare depicted as planar, and planar structures are most typical, othertopologies may be utilized. That structure 50 uses two metal layers 52,54 in the CMOS stack as electrodes while etching the metal layer inbetween to form the cavity which is then filled with theenvironment-sensitive dielectric material 56. The capacitance betweenthe two metal layers 52, 54 is then measured to determine amount of theselected chemical that has been absorbed by the environment-sensitivedielectric material 56. The series oxide capacitances 58, 59 that areleft above and below the environment-sensitive dielectric material layer56, due to the inter-metal dielectrics, reduce the sensitivity of theenvironment-sensitive capacitor. However, as shown in the structure 100illustrated in FIG. 2 b, in many CMOS processes, the metal layersconsist of a core metal layer of one composition disposed between twometal adhesion layers 102, 104 of a different composition. In thedescribed method, by using a selective etch that dissolves the coremetal layer and not the metal adhesion layers 102, 104, the core metallayer can be removed leaving a cavity between the two metal adhesionlayers 102, 104 that is then filled with the environment-sensitivedielectric material 106. The metal adhesion layers 102, 104 act as thetop and bottom electrodes of the environment-sensitive capacitor. Metalvias 108 are shown connecting the metal adhesion layers 102, 104 toother metal layers 110, 112 in the CMOS that act as conductive leads towire the sensor to interface circuitry or connect the capacitorstructure to other electronic structures within the same CMOS structure.As would be appreciated by those of skill in the art, in order toeffectively integrate a capacitor into an electrical circuit, theelectrodes of the capacitor are connected to conductive leads, which inthe embodiments described herein are other metal layers of the CMOSstack, but can be integrated by other processes and structures andcomprise other structures that are either designed into the CMOS stackbefore the capacitive sensor structure is formed, or afterwards.

This method of fabricating an environment sensor integrated into acomplementary metal oxide semiconductor chip will now be described inmore detail. The method described herein, in a general sense, isperformed on a CMOS stack that comprises a CMOS dielectric, and inorder, and when the CMOS stack comprises a substrate, in order ofincreasing distance from the substrate, a first metal layer, a secondmetal layer and a third, discontinuous metal layer, where the secondmetal layer optionally comprises a core metal layer disposed between twometal adhesion layers. The third metal layer is discontinuous, meaningthere are a plurality of (two or more) CMOS dielectric-filled gaps inthe layer. The first, second and third metal layers are separated byCMOS dielectric, and according to convention, for example and withoutlimitation, may be described as m1 (metal 1), m2 (metal 2) and m3 (metal3), respectively, with the first metal layer being at the “bottom” ofthe CMOS stack and when a substrate is present, closest to the substrateof the CMOS stack, and the third metal layer being at the “top” of theCMOS stack, closest to a surface of the CMOS stack. No directionality ororientation is implied by the designation of a “top” or “bottom”, whichare only designated as such according to how such structures aretypically depicted. Additional metal layers may be present in the CMOSstack, on either side of the first, second and third layers, or even inbetween the layers, meaning the first, second and third metal layers maybe referred to, for example, as m2, m3 and m4, respectively, m3, m4 andm5, respectively, m2, m4 and m5, respectively etc., depending on theconfiguration of the CMOS stack The first and second metal layers may becontinuous or discontinuous. The first and third layers, which may ormay not, but typically do, comprise a core metal layer disposed betweenmetal adhesion layers, are independently electrically connected toadjacent metal adhesion layers of the second metal layer using one ormore metal conductors, such as vias.

The dielectric of the CMOS stack is etched to provide a channel, andtypically two or more channels extending from the surface of the CMOSstructure to expose the core metal layer of the target metal layer. Ofnote, though not necessarily relevant to the methods described herein orthe function of the sensor, the second and third metal layers alsotypically comprise a core metal layer of one composition disposedbetween two metal adhesion layers of another composition.

A CMOS structure prepared by any available method, including theTower/Jazz Semiconductor 0.35 μm process, may be used as a startingmaterial in this process. The metal layer to be converted to a capacitortypically comprises a core metal layer of one composition disposedbetween two metal adhesion layers of another composition. A CMOSprovides a complex electronic circuit contained within a very smallarea. As an example, as shown in FIG. 3, a typical CMOS stack 200 (chipor otherwise, that is typically commercially available) has severaldiscontinuous metal layers 210, 212, 214, 216 that provide paths forelectricity to pass through the electrical circuit contained within thechip. It should be noted that FIGS. 2-5 are schematic in nature and arenot in proportion, and also do not depict the entire CMOS stack, butonly relevant portions necessary to illustrate the embodiments of themethods and devices described herein. The metal layers 210, 212, 214,216 are disposed within a matrix of CMOS dielectric 218 and layers 210,212 and 214 are discontinuous in the sense that they do not cover thesame area as continuous metal layers, in that they (in reference toFIGS. 2-5) have lateral gaps of CMOS dielectric that extend through orterminate those metal layers. Metal layer 216, while shown as acontinuous layer in FIG. 3, may also be discontinuous. The metal layers210 and 212 are shown to be electrically-connected to metal layer 216through metal vias 208 which are metal connections providing a path forthe flow of electricity from one metal layer to another. Metal layers210, 212, 214, 216 of the CMOS structure each comprise a core metallayer 220 of one composition disposed between two metal adhesion layers202, 204 of another composition. The core metal layer 220 may be of Almetal or alloys, Cu metal or alloys, or any other metal or alloy knownto be suitable for use in a CMOS structure as a core metal layer. Theadhesion layers 202, 204 may be Ti, W, Ta, or a combination thereof orany other metal or alloy known to be suitable for use in a CMOS as anadhesion layer. The CMOS dielectric 218 may be SiO₂ or low-k dielectricssuch as Novellus fluorinated oxide (FSG); or other suitable low-kdielectrics, including without limitation fluorine-doped or carbon-dopedSiO₂.

The CMOS dielectric is selectively etched, for example by anisotropicwet etching or dry etching, to provide at least two channels extendingfrom the surface of the semiconductor to an intermediate target metallayer. Selectively etching provides discrete, specifically placedchannels through the CMOS dielectric. The etchant is chosen to only etchthe CMOS dielectric and not the metal layers, thus, providing onlyremoval of the CMOS dielectric. The position of the channels can bedictated by the placement of discontinuities in the masking, uppermostmetal layer of the CMOS structure, or by any other suitable maskingmeans. In addition to being selective for CMOS dielectric andsubstantially inert with respect to the metal layers, the etchant ispreferably an anisotropic oxide etchant. Anisotropic oxide etchantsselectively etch the CMOS dielectric depending on the crystalorientation, etching different crystal orientations at different rates.This effect allows the CMOS dielectric to be etched in substantially onedirection, in this case, perpendicular to the metal layers from thesurface to the intermediate target metal layer, without substantiallyetching in other directions, in this case, parallel to the metal layers.An example anisotropic oxide etch is reactive ion etching with a mixtureof CHF₃ and O₂ gases. Etching of the oxide is continued only until thechannels extend to expose the core metal layer of the target metallayer. Dry etching methods may be used to produce the passages andexpose the core metal layer of the target metal layer, though dryetching processes substantially increase processing costs as compared towet etching.

Next, the core metal layer is selectively etched, leaving behind acavity between the metal adhesion layers. The etchant is chosen to onlyetch the core metal layer and not the metal adhesion layers or the CMOSdielectric, only removing the core metal layer. For an Al core layer andTiW adhesion layers, an example etchant would be Transene AluminumEtchant A, a mixture containing 80% phosphoric acid, 5% nitric acid, 5%acetic acid and 10% distilled water. For a Cu core layer with tantalumadhesion layers, an example etchant would be nitric acid. In this way,an open cavity is formed between the metal adhesion layers. However, incertain embodiments, the intermediate layer is discontinuous, with CMOSdielectric passing though the metal adhesion layers and the core metallayer such that the CMOS dielectric which fills the discontinuities ofthe target intermediate metal layer remains creating CMOS dielectricpillars that provide support to help prevent the cavity from collapsingduring subsequent processing. By use of the term “pillars” no geometricor topological shape or configuration is implied.

Finally, the cavity between the metal adhesion layers, and optionallythe channels in the CMOS dielectric, are filled with anenvironment-sensitive dielectric material, e.g., capable of selectivelyabsorbing the chemical to be sensed. It should be noted that the sensorwill function for certain purposes without filling the cavity, leavingan air-gap as an environment-sensitive dielectric. Theenvironment-sensitive dielectric material is a material for whichabsorption of or contact with a specific chemical, or exposure tocertain physical conditions, alters its permittivity, such that when itis used as a dielectric in a capacitor, the capacitance of the capacitorwill be changed depending on the amount of the specific analyte to whichthe environment-sensitive dielectric material is exposed. By monitoringthis change in capacitance, the amount of the specific analyte beingsensed can be determined. In this case, the capacitor consists of themetal adhesion layers with the environment-sensitive dielectric materialdisposed therebetween.

In alternative embodiments, the target metal layer either only comprisesa single core metal layer or the etching removes both the core metallayer and the adhesion layers. In these embodiments, theenvironment-sensitive dielectric material is directly electricallyconnected to the other metal layers by the metal vias to produce acapacitive sensor.

The systems, apparatuses and devices described herein are fabricated asmicrofabricated devices (referred to herein as “microdevices” or“microsystems”, referring generally to the small size of such systems,devices or apparatuses, and not inferring micrometer-scale ornanometer-scale dimensions). MEMS (microelectromechanical systems) orNEMS (nanoelectromechanical systems), comprising micron- ornanometer-scale mechanical parts/structures) devices are microdevices.Microfabrication methods and compositions useful for preparing thesystems, apparatuses and devices described herein are well-known in theMEMS, NEMS, printed-circuit board (PCB) and integrated circuit (IC)manufacturing industries. Microsystems may be manufactured from avariety of materials. Common materials include silicon (e.g.polycrystalline silicon and silicon nitride), glass, carbon (e.g. carbonnanotube and graphene), diamond, polymers and metals. A variety ofmethods may be used to manufacture the apparatuses (See, e.g., G.Fedder, MEMS Fabrication, in Proceedings of the IEEE International TestConference (ITC '03), Sep. 30-Oct. 2, 2003, Charlotte, N.C.; H. Baltes,et al., CMOS-MEMS, Wiley-VCH, ISBN 3257310800, January 2005).

The devices described herein can be prepared according to standard MEMS,IC, PCB, etc. design and manufacturing methods and criteria. Electroniccircuits can be integrated into the device according to known methods.For example, the CMOS structure as described herein optionally includescircuitry for capacitance measurement, including, for example andwithout limitation, one or more electrical or electronic elements, suchas amplifier(s), preamplifier(s), capacitive divider circuit(s) andcapacitive bridge circuit(s). The devices can be packaged in anysuitable manner providing for efficacy of the sensors and overallfunction of the devices.

Thin films are deposited by any of a variety of methods, for example andwithout limitation: physical vapor deposition (PVD), such as sputteringand evaporation; and chemical deposition, such as chemical vapordeposition (CVD), including low pressure CVD and plasma enhanced CVD,and thermal oxidation. Exemplary methods for patterning such devicesinclude: mask lithography (photolithography), electron beam lithography,ion beam lithography, X-ray lithography, diamond patterning, injectionmolding, microstereolithography, silicon surface micromachining, highaspect ratio silicon micromachining and silicon bulk micromachining maybe utilized.

Structures may be formed by etching, including wet and dry etchingmethods, which may be isotropic or anisotropic methods. Wet methodsinclude, without limitation: acid etching (e.g., with hydrofluoric,nitric, and/or hydrochloric acids), peroxide etching, basic etching(e.g., with potassium hydroxide) and electrochemical etching. Dryetching methods include, without limitation: vapor etching, includingxenon difluoride etching, plasma etching, including reactive ion etchingand deep reactive ion etching (e.g., etching of silicon-on-insulator(SOI) and epitaxial silicon and single crystal reactive etch andmetallization (SCREAM) methods). CMOS structures/processes may beutilized in conjunction with the MEMS manufacturing methods listed above(see, e.g., G. Fedder, CMOS Based Sensors, in Proceedings of the IEEESensors Conference (IEEE Sensors '05), pp. 125-128, Oct. 31-Nov. 3,2005, Irvine, Calif. and G. K. Fedder, Sensors & Actuators A, vol. 57,no. 2, pp. 103-110, November 1996). Both wet and dry etching methods maybe isotropic or anisotropic.

Inkjet printing, for example inkjet printing methods using polymerdissolved in solvent, also can be used to deposit and pattern films(see, e.g., Alfeeli B., et al. Solid State Sensors, Actuators andMicrosystems Workshop Hilton Head Island, S.C., Jun. 1-5, 2008, pages118-121, for inkjet deposition of Tenax TA). Given the significantnumber of materials, methods and structural/topological variationspossible, a person of skill in the field of microfabrication ofmicrodevices (e.g., MEMS, NEMS and IC devices) may use any of a varietyof methods and materials to produce/manufacture the microdevicesdescribed herein. U.S. Pat. Nos. 6,171,865, 6,850,859, 7,061,061 and7,338,802, each of which is incorporated herein by reference for itstechnical disclosure, describe MEMS sensor systems, methods ofmanufacturing such systems, implementation and use of such systems.

Environment-sensitive dielectric material can be added by using a customdrop-on-demand inkjet system (L. Weiss, et al. Inkjet deposition systemwith computer vision-based calibration for targeting accuracy, Technicalreport CMU-RI-TR-06-15, Carnegie Mellon University, March, 2006) todeposit the polymer in solution. As would be appreciated by those ofordinary skill in the art of CMOS fabrication techniques and, moregenerally, semiconductor structure fabrication techniques, themanufacturing steps may include additional steps, and further, identicaland substantially or essentially identical structures may be fabricatedby a variety of methods, which would become apparent in light of theteachings of this disclosure.

The environment-sensitive dielectric material may be a chemicallysensitive polymer or may be a non-polymer. Chemically sensitive polymerscan be used to detect most volatile and semi-volatile compoundsincluding industrial solvents, liquid fuels, many chemical warfareagents, and certain volatile compounds present in commercial explosives.In addition, polymer-based sensors have been used to detect severalnon-VOC compounds, including water vapor and ammonia. The utilizedenvironment-sensitive dielectric materials can more broadly include anysubstance that can be delivered within a solvent or made into a lowviscosity solution and thus wicked in between the metal adhesion layerssurrounding the cavity. Some examples other than polymers includenanoparticles (functionalized or non-functionalized) suspended insolution, sol-gel materials and dielectric oils.

A large number of polymers, including polyimide, polymethyl methacrylate(PMMA), poly(ethylene teraphthalate) (PET), polysulfone (PSF), celluloseacetate butyrate (CAB), polyethynyl fluorenol (PEFI), have been used forhumidity application. Polymers may be selected for their ability toselectively form weak reversible chemical interactions (hydrogen bonds,van der Waals bonds, and dipole-dipole interactions) with a particularanalyte. Liquid polymers or polymers with a low modulus of elasticitymay be preferred in certain instances because they absorb analytes morequickly than rigid polymers (S. V. Patel et al. Chemicapacitivemicrosensors for volatile organic compound detection Sensors andActuators B 96 (2003) 541-553). Polymers may be chosen based on thefindings in the literature applying solubility parameters. Other factorsin choosing the polymers include stability, ease of acquisition,solubility in a suitable solvent and ease of coating application (S. V.Patel et al. Sensors and Actuators B 96 (2003) 541-553). U.S. Pat. No.5,970,315, incorporated herein by reference for its technicaldisclosure, also provides a list of analytes some polymers are sensitiveto.

As an example, the fluoroalcohol SXFA has an affinity forhydrogen-bonding bases and is useful for the detection of chemicalwarfare agents, such as Sarin. Dicyanoallyl silicone (e.g., OV-275,20,000 cSt) and cyanopropyl methyl phenylmethyl silicone (e.g., OV-225,9,000 cSt), are siloxane-based compositions that can be used to detectbyproducts and impurities found in explosives.Polyethylene-co-vinylacetate (PEVA, e.g., 40% acetate content),polyepichlorohydrin (PECH, e.g., 700,000 MW), polycarbonate urethane(PCUT), polyisobutylene (Pm, e.g., 1350 MW), and polydimethyl siloxane(PDMS, e.g., 100,000 cSt(centistokes)) may be used to detect volatileorganic compounds (VOCs) with moderate to low polarity values (S. V.Patel et al. Sensors and Actuators B 96 (2003) 541-553). Other usefulpolymeric materials include, poly(dimethyl siloxane (PDMS) andpoly(etherurethane) (PEUT).

Nanocomposites, such as nanocluster and nanocrystalline materials, alsoare useful as chemical-sensitive dielectric materials in the capacitordevices described herein. Nanocomposites contain nanometer-sized (e.g.1-100 nm in at least one dimension) particles of any suitablemorphology, such as metallic or ceramic particles. Examples of suitablenanocluster materials include, without limitation, silicon nanoclusters,metal nanoclusters and gold nanoclusters. Nanoclusters and nanoparticlescan be capped with a variety of thiol groups, for example that containalkane, alkene or other carbon containing moiety. In one example, WeiYao, et al. disclose a Gold-PVA nanocomposite that is useful formoisture sensing. (A capacitive humidity sensor based on gold-PVAcore-shell nanocomposites Sensors and Actuators B 145 (2010) 327-333).In another example, silica nanoparticle compositions, such as mesoporoussilica (e.g. 2-50 nm pore diameter) or aerogels, have found use aschemical-sensitive dielectric materials in capacitance sensors fordetecting humidity and VOCs. An aerogel is a mesoporous ceramicmaterial. Silica in the form of aerogel has a highly porous structuremainly consisting of mesopores supported by a nanoparticle cross-linkingframework (Chien-Tsung Wang et al. Humidity sensors based on silicananoparticle aerogel thin films Sensors and Actuators B 107 (2005)402-410).

Non-limiting examples of environment-sensitive dielectric materialsinclude: polyimide, polyisobutylene, polydimethyl siloxane,polycarbonate urethane, polyethylene-co-vinylacetate, siloxanefluoroalcohol, polyepichlorohydrine, cyanopropyl methyl phenylylmethylsilicone, and dicyanoallyl silicone.

Also provided herein is a CMOS-based sensor, fabricated by the methodsdescribed herein or otherwise. The sensor comprises, a first metallayer; a second layer comprising an environment-sensitive dielectricmaterial; a third metal layer, wherein the first and third metal layersare electrically connected to the environment-sensitive dielectricmaterial, producing a capacitive sensor; and one or more passagesextending from an outer surface of the sensor to the second layer,wherein the one or more capacitive environment sensors are integratedinto circuitry for capacitance measurement. The environment-sensitivedielectric material layer is optionally disposed between two metaladhesion layers (that is, derived from adhesion layers of a CMOS stack).The first and third metal layers are electrically connected either tothe environment-sensitive dielectric material or, when present, to anadjacent metal layer of the second layer to form a capacitive sensor(meaning the first and third layers are not directly connected to eachother, but connect to structures, such as vias or metal adhesion layersin contact with the environment-sensitive dielectric material and whichserve as electrodes of a capacitor). The sensor comprises one or morepassages extending from an outer surface of the sensor to theenvironment-sensitive dielectric material layer. All of the features ofthis chemical sensor have been described above.

As described above, in one embodiment, the metal adhesion layers isabsent from the second layer, and the environment-sensitive dielectricmaterial layer is then directly electrically connected to each of thefirst and third metal layers by metal vias.

A system for environment sensing also is provided having one or morechemical sensors integrated into a complementary metal oxidesemiconductor structure. The system comprises a capacitive sensor,according to any embodiment described herein, incorporated intoelectronic circuitry that supports the sensor in its use (circuitry forcapacitance measurement). More than one capacitive sensor and/orelements of the circuitry for capacitance measurement are integratedinto the same CMOS structure as the capacitive sensor according tocertain embodiments. In one example, the circuitry into which the one ormore chemical sensors are integrated is a charge-based capacitancemeasurement (CBCM) circuit (See, e.g., Y. Chang, Y., et al. Charge-BasedCapacitance Measurement for Bias-Dependent Capacitance, IEEE ElectronDevice Letters, Vol. 27, No. 5, May 2006, pp. 390-392).

In one embodiment of the described method, the fabrication of the CMOSwith an integrated chemical sensor begins with a CMOS chip having atleast three discontinuous metal layers 310, 312, 314, 316 as shown inFIG. 4 a. The metal layers are composed of a core metal layer 320disposed between two metal adhesion layers 302, 304. In thisnon-limiting embodiment, the CMOS chip may be fabricated using theTower/Jazz Semiconductor 0.35 μm process or any process capable ofproducing a CMOS chip having at least three metal layers composed of acore metal layer disposed between two metal adhesion layers. As shown inFIG. 4 b, an anisotropic oxide etch capable of dissolving the CMOSdielectric 318 is used to selectively etch channels 322 into the CMOSdielectric 318 that extend to an intermediate metal layer 316 in theCMOS that is connected in series between two other metal layers 310, 312by metal vias 308. The channels 322 are placed such that they do notpass through any metal layers and are etched to a depth that justexposes the top of the intermediate metal layer 316. The channels 322may appear as a series of holes on the surface of the CMOS chip and maybe of uniform size or differ in size. This selective etching of thechannels 322 may be accomplished by using the top-most metal layer as anetch mask, defining the channels in the CMOS dielectric.

As shown in FIG. 4 c, after the channels have been placed in the CMOSchip, an etchant chosen to selectively dissolve the core metal 320 whileleaving the metal adhesion layers 302, 304 intact is injected into atleast one of the channels 322 and the core metal 320 is dissolved fromthe intermediate metal layer 316 forming a cavity 324. A photoresist,for example, Clariant AZ-4210, is used to protect the exposed core metalpads (not shown) at the surface of the CMOS from being dissolved duringthis process. Metal layer 310 and metal layer 312 are enclosed withinthe CMOS dielectric 318 defined by metal layer 314, and are not exposedto the etchant. The chip is rinsed in de-ionized water followed bysuccessive rinses in acetone and methanol to prevent surface tensionfrom pulling the capacitor plates (metal adhesion layers 302, 304)closed.

FIGS. 5 a and 5 b are schematic drawings showing two different expandedcross-sections of a CMOS capacitive sensor structure made according tothe described method wherein the intermediate metal layer was alsodiscontinuous. Reference numbers are the same as in FIG. 4. As shown inthe cross-section of FIG. 5 b, because the intermediate metal layer inthe CMOS is discontinuous and the discontinuities are filled with theCMOS dielectric 318, CMOS dielectric pillars 326 will be left in thecavity 324 formed when the core metal 320 has be removed fromintermediate metal layer 316. These CMOS dielectric pillars 326 providesupport for the cavity 324. The CMOS can be designed such that thediscontinuities are specifically located to create CMOS dielectricpillars 326 in specific locations of the CMOS to provide the necessarysupport to keep the cavity 324 open, for example, in the areas about aperiphery of at least one of the channels 322.

As shown in FIG. 4 d, an environment-sensitive dielectric material 306is then injected, wicked or otherwise deposited into at least one of thechannels 322 until the cavity 324 formed by the removal of the coremetal 320 and, optionally the channels 322, are filled with theenvironment-sensitive dielectric material 306 such that at least aportion of the environment-sensitive dielectric material 306 is exposedto the atmosphere that the chemically sensitive capacitor will be usedto analyze through the channels 322, which extend to the outer surface328 of the structure.

In an alternative embodiment of the described method, shownschematically in FIG. 6, the fabrication of the CMOS structure with anintegrated chemical sensor begins with a CMOS chip having at least threemetal layers 410, 412, 416. FIGS. 6 a, b and c depict only a portion ofa larger CMOS structure. In those figures, metal layer 410 is depictedas discontinuous. Metal layers 412 and 416, while depicted as continuousto the extent of the portion of the CMOS structure shown, may becontinuous or discontinuous. The intermediate metal layer 416 iselectrically connected to metal layers 410, 412 disposed on oppositesides of intermediate metal layer 416 by an array (large number) ofmetal vias 408. The metal layers are composed of a core metal layer 420disposed between two metal adhesion layers 402, 404. As shown in FIG. 6a, an anisotropic oxide etch capable of removing the CMOS dielectric 418is used to selectively etch channels 422 into the CMOS dielectric 418that extend to the intermediate metal layer 416.

As shown in FIG. 6 b, after the channels 422 have been placed in theCMOS chip, an etchant is used to selectively dissolve both the coremetal 420 and the metal adhesion layers 402, 404 forming a cavity 424.The chip is rinsed in de-ionized water followed by successive rinses inacetone and methanol.

As shown in FIG. 6 c, an environment-sensitive dielectric material 406is then injected, wicked or otherwise deposited into at least one of thechannels 422 until the cavity 424 formed by the removal of metal layer416 and, optionally the channels 422, are filled with theenvironment-sensitive dielectric material 406 such that at least aportion of the environment-sensitive dielectric material 406 is exposedto the atmosphere that the chemically sensitive capacitor will be usedto analyze through the channels 422, which extend to the outer surface428 of the structure. Although the devices depicted in FIGS. 2-6 depicta substrate the bottom-most layer, e.g., reference number 201 in FIG.3—the substrate is an optional component, though it is common in CMOSstructures.

It will be recognized that the procedure described above can also beutilized with a CMOS having metal layers with no metal adhesion layers.

More than one chemical sensor, for example and without limitation one ormore additional capacitive sensors as described herein, can beintegrated into the same CMOS structure. The sensors described hereincan be used for any environmental testing, so long as the capacitance ofthe sensor changes over time with exposure to an environmentalcondition, such as humidity, temperature, chemicals, CO₂, etc. Thedevice may be used to sense chemical(s) within a mask, such as abreathing mask, a gas mask or a respirator.

The devices described herein can be prepared according to standard MEMS,IC, PCB, etc. design and manufacturing methods and criteria. Electroniccircuits can be integrated into the device according to known methods.The devices can be packaged in any suitable manner providing forefficacy of the sensors and overall function of the devices. As shouldbe recognized by those of ordinary skill in the art, considerablevariation in the layout and components of such a device would result inequivalent functionality.

FIG. 7 illustrates one embodiment of a computer system for use inimplementing the sensor described above. The capacitive sensorsdescribed herein will comprise sufficient electronic circuitry to permitits intended use. At a minimum, and in reference to FIG. 7, electricleads are connected to the capacitor structures and to an “INPUT” of asensor computing device. The “INPUT” of sensor computing device maycomprise one or both of an amplifier (e.g., operational amplifier,preamplifier, differential amplifier, etc.) to amplify signal from thesensors and an analog-to-digital chip/circuit to convert raw analog dataobtained from sensors to a digital format. Signal received from sensorsis optionally converted to a digital signal, but conversion to a digitalsignal may be preferred in certain embodiments.

The “CONTROL” of sensor computing device comprises computer software(“software” (or computer software) includes, without limitation:application software, middleware, computer processes, programminglanguages, code, system software, operating systems, testware, firmware,device drivers, programming tools, data, etc. for carrying out aspecific task) and/or computer hardware. Useful computer software and/orhardware constituents are readily developed by those of ordinary skillin the related arts, such as using assembly language on amicrocontroller, or using any of a large variety of availableprogramming resources, languages, for example and without limitation: C,Matlab and Java.

The sensor computing device comprises a central processing unit (“CPU”)and “MEMORY” which stores data collected and any useful computersoftware (e.g. firmware) for obtaining, converting, analyzing, storingand uploading data. Memory may comprise of any useful data storagedevice, including ROM, PROM, FPROM, OTP NVM, RAM, EEPROM, flash memory,etc. Because the device in many instances is miniaturized, the memorycomponent is, to the extent possible, miniaturized. The CPU can compriseof any useful processing circuitry, chip(microprocessor)/hardware/software, combinations etc. The sensorcomputing device also typically comprises an input/output interface(“I/O” or communications interface), such as a wired interface such as asuch as a USB (e.g., USB 2.0), Ethernet, serial (e.g. RS232), GPM(General Purpose Interface Bus, e.g. IEEE-488), or firewire interface,or a wireless interface, such as an IEEE 802.11 (e.g., 802.11(a),802.11(b), 802.11(g) or 802.11(n) interface), a Bluetooth interface oran RFID-based interface for communicating with an external device, whichcan be a second computing device (e.g., PC, laptop, smartphone, PDA,tablet PC, iPad, etc.) for uploading data, analyzing data, outputtingdata, downloading firmware to the device, or for any activity. Devicecan be powered by batteries, such as rechargeable batteries (e.g., via aUSB interface) or any suitable power source.

The sensors described herein are suitable for use in a remote sensor,such as for monitoring analyte levels in a gas mask or filtration deviceto determine the presence of environmental contamination, the status ofadsorbent levels in the mask (indicating breakthrough of, e.g., VOCs),or to monitor a subject's respiration. As such, the miniaturized devicecan be installed in a gas mask, and analyte levels can be monitored inthe manner indicated and the results stored within the system memory.Periodically or continually the data can be uploaded to an externalcomputing device for monitoring, analysis, storage, etc.

Signal received from sensors is optionally converted to a digitalsignal, but conversion to a digital signal may be preferred in certainembodiments. In certain embodiments, the signal from sensor is comparedto stored data by differencing the output of the sensor either by analogor digital processing. Where data is obtained remotely and transferredto a second computer, the differencing or other comparison methods canbe performed either at the sensor computing device or external device.It may in many instances be in such a configuration to conduct thedifferencing at the sensor computing device, though if a real-timeconnection (typically wireless, including substantially real-timeconnection, meaning that data is transferred from the sensor device tothe external device regularly, such as every second, 10 seconds, minuteor even hourly or daily depending on system tolerances) between thesensor and external device is used, differencing and comparison againstreference data, if used, can be conducted in one or both of the sensorand external devices in a stand-alone or distributed manner. Alarmfunctions, indicative of analyte levels reaching a desired threshold,may be programmed or otherwise incorporated into the devices describedherein to provide a discernable signal indicating crossing of athreshold. One non-limiting example of such a threshold is an increasein analyte concentrations in a gas sample indicative of VOC breakthroughin a gas mask indicative of loss of function of an adsorbent material inthe gas mask.

Example

A 300 μm diameter circular parallel-plate sensor was designed andfabricated using the method described above. The starting CMOS had fourmetal layers 310, 312, 314, 316, as shown in FIG. 4 a and was fabricatedusing the Tower/Jazz Semiconductor 0.35 μm process. In reference toFIGS. 4 a, the metal layers 310, 312, 314, 316 of the CMOS structureeach had a core metal layer 320 of Al disposed between two metaladhesion layers 302, 304 of TiW, and the CMOS dielectric 318 was SiO₂.In addition, intermediate metal layer 316 was also discontinuousalthough this is not shown in FIGS. 4 a-4 d. An anisotropic oxide etch,reactive ion etching with a mixture of CHF₃ and O₂ gases, was used toform channels 322 through the CMOS dielectric and down to intermediatemetal layer 316 connected in series to two other metal layers 310, 312by metal vias 308 (FIG. 4 b). The exposed aluminum core metal layer 320of the intermediate layer 316 was then wet etched using TranseneAluminum Etchant Type A, a mixture containing 80% phosphoric acid, 5%nitric acid, 5% acetic acid and 10% distilled water, injected into oneof the channels, leaving the TiW adhesion layers 302, 304 intact (FIG. 4c). Photoresist (Clariant AZ-4210) was used to prevent the exposedaluminum pads (not shown) on the surface of the CMOS from being etchedduring this stage. The chip was rinsed in de-ionized water followed bysuccessive rinses in acetone and methanol to prevent surface tensionfrom pulling the capacitor plates (the TiW adhesion layers 302, 304)closed. A polymer 306 composed of polyimide (HD Microsystems PI-2556)diluted by a factor of 24:1 with a 50:50 mixture ofn-methyl-2-pyrrolidone and methoxy propanol (HD Microsystems T-9039) wasinjected into one of the channels 322 using a custom drop-on-demandinkjet system (FIG. 4 d). Polyimide was chosen because it is widely usedfor humidity sensing, allowing comparison with other sensor topologies.

Specifically as depicted in FIG. 5 b, an 80 μm diameter channel 322 a inthe center of the structure served as a target for drops from a 60 μminkjet nozzle. Multiple square channels 322 b (2.6 μm on a side) servedboth as release holes for the aluminum etch and as access holes forwater vapor to absorb into the polyimide layer. Discontinuities in theintermediate metal layer 316 resulted in CMOS dielectric pillars 326surrounding the channel 322 a at the center of the device, holding thetop and bottom capacitor plates (TiW adhesion layers 302, 304) apart.

SEM photomicrographs of the surface of the CMOS before and afterinkjetting of the polyimide are shown in FIGS. 8 a and 8 b. Because thepolyimide fill could not be verified optically during inkjetting, thecavity 324 and channels 322 a, 322 b were deliberately overloaded,resulting in significant polymer residue on the outer surface 328 of theCMOS. This residue layer will reduce the speed of the device slightly,since water vapor will need to diffuse a longer distance into the deviceto reach the polymer 306 in the cavity 324. However, the process can beoptimized to minimize the excess polymer and improve the response timeby methods know to those skilled in the art.

Theoretical Model:

To assist in understanding the described chemical sensor, information onthe theoretical model that underlies the described chemical sensor isprovided, with reference to its use as a humidity sensor. The humiditysensor is modeled using a parallel-plate capacitor approximation, sincethe gap between the electrodes (metal adhesion layers) (nominally 450nm) is very small relative to the area of the structure. The dielectricconstant for polymer with absorbed water vapor is:

∈=[γ(∈_(H) ₂ _(O) ^(1/3)−∈_(P) ^(1/3))+∈_(P) ^(1/3)]³  Equation 1

where γ is the volume fraction of water in the film, and ∈_(p) and∈_(H2O) are the dielectric constants of polymer and water, respectively(Fenner and Zdankiewicz, “Micromachined Water Vapor Sensors: A Review ofSensing Technologies”, IEEE Sensors J., 2001, Vol. 1, pp. 309-317).

In this case for humidity sensing, polyimide was used due to its linearresponse to relative humidity. Polyimide has a volume fraction of waterapproximately given by (Shibata, cited in Background section):

γ=c ₁(% RH)^(c) ²   Equation 2

The terms c₁ and c₂ are temperature-dependent constants and % RH is thepercentage of relative humidity.

The sensor is modeled as a parallel-plate capacitor,

$\begin{matrix}{C = \frac{ɛ\; A}{d}} & {{Equation}\mspace{14mu} 3}\end{matrix}$

where A is the area and d is the gap between the capacitor plates. Thiscapacitance can change due to either changes in the dielectric constantaccording to equation 1, or due to changes in capacitor dimensions frommaterial swelling. Polymer expansion is neglected in the humidity sensormodel because the volume coefficient of expansion of polyimide is small,only 60-75 ppm/% RH.

Testing Circuit:

The exemplar humidity sensor using polymer polyimide was integrated on aCMOS die with a charge-based capacitance measurement (CBCM) circuitshown in FIGS. 9 a-9 c. Those skilled in the art will recognize thatmany other circuits exist to detect capacitance for a sensor, includingswitched capacitor circuits, integration amplifiers, transimpedanceamplifiers, and high-impedance voltage preamplifiers. Likewise thoseskilled in the art will recognize that the capacitive sensor can becombined with one or more other capacitive sensors or fixed capacitorsto form capacitive divider circuits and capacitive bridge circuits,which can improve the performance of the output offset and help rejectcommon-mode disturbances.

This CBCM circuit uses two separate measurements to measure the sensingcapacitance C and isolate it from the parasitic capacitance to ground,C_(p). In the current design, this parasitic capacitance results fromthe capacitance between the top electrode plate (top metal adhesionlayer) and the grounded adhesion layer left behind from the metal layerused to protect the wiring from the aluminum etch, and is approximately10% of the size of the sensing capacitance. This capacitance may bereduced by wiring masking metal to the top electrode plate of thesensor. In the first measurement, illustrated in the timing diagram ofFIG. 9 b, the Clk input is grounded while the center node V_(c) is firstdischarged to ground, then charged to V_(dd), resulting in both thesensing capacitance C and the parasitic capacitance C_(p) being charged.This cycle is repeated at frequency f_(s) and measured with a DCpicoammeter, resulting in the current:

I ₁ =f _(s)(C+C _(p))V _(dd)  Equation 4

In the second measurement (FIG. 9 c), the Clk input is grounded whilethe center node is discharged, but brought to V_(dd) when the centernode is charged. Charge is only stored on the parasitic capacitanceC_(p), since there is no voltage drop across the sensing capacitor,resulting in total current:

I ₂ =f _(s) C _(p) V _(dd)  Equation 5

The sensing capacitance is calculated as:

$\begin{matrix}{C = \frac{I_{1} - I_{2}}{f_{s}V_{dd}}} & {{Equation}\mspace{14mu} 6}\end{matrix}$

Flow System:

The exemplar humidity sensor was tested using the flow system shown inFIG. 10. A Milligat M6 solvent pump pumps liquid water into a 1 L/minnitrogen flow; a brass block heated to 48° C. is used to encourage thewater to vaporize. The airflow passes into a 250 mL mixing volume, andthen into the test box containing the sensor. A Honeywell HIH 4000humidity sensor in the test box is used to obtain actual referencehumidity value.

Humidity Testing:

The exemplar humidity sensor using polymer polyimide was measured for arange of relative humidity from 0% to 40%, resulting in the responsecurve in FIG. 11. The results presented herein are representative ofthis embodiment, but do not represent the limits of the results that areattainable with other embodiments. The sensor response was fitted to thetheoretical model for polyimide, giving an adjusted R² value of 0.9964.The sensitivity in the linear region of the sensor was fitted to be0.31% change in capacitance per percent relative humidity. A secondsensor was tested using a Miller-Nelson HCS-401flow-temperature-humidity control system over the range from 30% to 75%relative humidity, giving a linear response with a sensitivity of0.30%/% RH. The non-integrated polyimide vertical parallel-plate sensorpresented in Dokmeci and Najafi (cited in the Background of the Artsection) had a comparable sensitivity of 0.31%/% RH.

Response Time:

In the CMOS, the environment-sensitive dielectric material (polymer)wicks to the top of the channels, as shown schematically in FIGS. 5 aand 5 b. As a result, water vapor must diffuse through several micronsof absorbing material (polymer) before reaching the area between the twometal adhesion layers that forms the actual capacitor, potentiallyresulting in a slow response time. Polymer deposited on the top surfacewill further increase this distance, slowing down the device further.The absorption of water vapor into the polymer is modeled using Fick'ssecond law:

$\begin{matrix}{\frac{\partial c}{\partial t} = {D{\nabla^{2}c}}} & {{Equation}\mspace{14mu} 7}\end{matrix}$

where c is the concentration of water vapor and D is the diffusioncoefficient of water vapor for the specific polymer. This diffusionmodel was found to be accurate for relative humidity pulses up to 50%.The response time of the sensor was simulated using finite elementmodeling. The diffusion coefficient for water vapor in polyimide is15×10⁻¹⁴ m²/s. FIG. 12 shows the concentration of water vapor betweenthe plates for a sensor having polymer to the top of the release holesfor a 3% step in relative humidity, corresponding to 0.038 mol of waterper cubic meter for 25° C. temperature and 1 atm pressure. The simulatedtime constant for the device is 68 s.

To determine the actual response time of the sensor, a test was run withlow concentration pulses of water vapor. The absorption (FIG. 13 a) anddesorption (FIG. 13 b) responses of the integrated sensor to a pulse ofrelative humidity, along with the response of the reference sensor weredetermined. Unlike the simulated response, the input humidity pulse isnot a sudden step; the flow system used includes a mixing volume andlength of tubing that will result in a more gradual transition. However,the response time can be compared to the Honeywell reference sensor witha known time constant to changes in relative humidity of 15 s.

The measured rise and fall time constants of the integrated verticalparallel-plate sensor were both 3.16 minutes, while the Honeywellreference sensor had a rise time constant of 2.25 minutes and a falltime constant of 2.0 minutes. Although the specific polymer used in theHoneywell sensor is unknown, the desorption and absorption diffusioncoefficients of polymers can be different at some concentrations, whichlikely causes the difference between the rise and fall times for thesensor. The response time constant of the integrated sensor is 55 slonger than the response of the reference Honeywell sensor for a risingpulse, and 70 s longer for a falling pulse. The simulation gave aresponse 53 s longer (68 s response time constant for the sensorsimulation compared with a response of 15 s from the datasheet for theHoneywell sensor). Additional polymer on the top surface of the device,which was not included in the simulation, could account for a slightlylonger measured response time.

Noise:

Another important specification of a chemical sensor is the noiseperformance, i.e., the limit of detection or the minimum detectableconcentration of analyte a chemical sensor can measure. The noise in acapacitor is typically small, unless there are a large number of chargetraps in the device, so the dominant noise in the system will be fromthe CBCM circuit. One method for characterizing the noise response andlimit of detection of a sensor is the Allan variance:

$\begin{matrix}{\sigma_{C}^{2} = {\frac{1}{M}{\sum\limits_{k = 1}^{M}\; {\frac{1}{2}\left( {C_{k} - C_{k - 1}} \right)^{2}}}}} & {{Equation}\mspace{14mu} 8}\end{matrix}$

where M is the number of samples. C_(k) is the kth measured capacitancevalue, normalized by the initial capacitance C₀. The Allan deviation,the square root of the Allan variance, is a measure of thesample-to-sample variation of a sensor, and indicates the minimum changethat can be detected. The minimum detectable change in capacitance isthus given by:

ΔC=C _(O)√{square root over (σ_(C) ²)}  Equation 9

The Allan deviation of a system is dependent on the sampling period.Longer sampling times result in additional averaging, reducing higherfrequency noise. However, for long averaging times, sensor baselinedrift results in an increase in Allan deviation. FIG. 14 shows a plot ofthe Allan deviation of the sensor after temperature compensation for arange of sampling times. The Allan deviation of a picoammetermeasurement of a comparable current through a resistor is also shown,and is an estimate of the noise level of the picoammeter. The Allandeviation is relatively flat for low integration times, approximatelyequal to that of the resistor; for higher integration times, sensordrift due to imperfectly compensated temperature and aging dominate,resulting in a rising Allan deviation. The lowest Allan deviation was6.39×10⁻⁶ for an averaging time of 0.5 s, corresponding to a minimumdetectable signal of 0.06 fF and a limit of detection of 0.0023% changein relative humidity.

Temperature Response:

The sensitivity to temperature is an important specification for asensor that is exposed to the external environment. The temperatureresponse was measured by using a tape heater to heat the packaged chip.Thermal grease was used to attach a thermocouple to the package near thechip to verify the chip temperature. To investigate the response of thephysical structure, the thermal sensitivity was measured in a nitrogenflow to eliminate any change in capacitance due to water vapor desorbingupon a temperature change.

FIG. 15 shows the temperature response of the sensor. The temperaturesensitivity was measured to be 0.19% change in capacitance/° C. Thissensitivity is relatively large, 2.7 times larger than the releasedinterdigitated sensor with the same polymer presented in Lazarus (citedin the Background section). The high sensitivity to temperature may bebecause the low density of the CMOS dielectric pillars in the currentstructure allows the top capacitor plate to move slightly as temperaturechanges. Higher CMOS dielectric pillar density should hold the gapapart, reducing the thermal sensitivity.

Cross-Sensitivity:

This sensor may be used as part of a chemical sensor system to detecttoxic industrial solvents, particularly volatile organic carbons (VOCs).As a result, cross-sensitivity to commonly used solvents is an importantspecification of the sensor. The sensor was tested by pumping liquidsolvent into a nitrogen flow stream using the same test setup used forhumidity testing. The system was calibrated using a gas chromatograph toverify the analyte concentration.

Table 1 shows the sensitivity of the humidity sensor to five volatileorganic carbon vapors. Since a capacitive sensor is sensitive to thedielectric constant of absorbed analyte, three high dielectric constantalcohols (IPA, ethanol and methanol) were chosen, as well as acetone andtoluene. The vapor pressure of each analyte is also listed, and reflectsthe volatility of the individual analyte. The largest measured responsewas for the highest dielectric constant analyte, methanol, with asensitivity of 1.35×10⁻⁴% change in capacitance per part per million(ppm). Although these responses are not insignificant, the sensor isdesigned to be used in an array of different chemical sensors, allowingtechniques such as principle component analysis to be used todifferentiate between chemical analytes.

TABLE 1 Material properties and sensitivity to chemicals incross-sensitivity test Vapor Dielectric Pressure at Sensitivity AnalyteConstant 20° C. (Pa) (%/ppm) Toluene 2.39 2900 1.60 × 10⁻⁵ Acetone 20.724500 1.80 × 10⁻⁵ IPA 19.92 4340 7.50 × 10⁻⁶ Ethanol 24.51 5850 5.70 ×10⁻⁵ Methanol 32.65 12800 1.35 × 10⁻⁴

To summarize the testing, the measured results for the exemplarembodiment demonstrate a vertical parallel-plate capacitive humiditysensor successfully integrated with CMOS testing electronics. Thesensitivity of the sensor is 0.31%/% RH, an increase of 72% over thehighest sensitivity previously demonstrated for an integrated capacitivehumidity sensor (Lazarus et al., cited in the Background section).Further, the sensitivity matches the sensitivity for polyimidepreviously measured in a non-integrated vertical parallel-plate sensor(Patel et al., cited in the Background section). The sensor was found tobe relatively slow, with a time constant on the order of a minute, sincewater vapor must diffuse several microns to reach the active part of thesensor. The limit of detection of the sensor was found to be 0.0023%relative humidity.

Although the present method, apparatus, and system have generally beendescribed in terms of specific embodiments and implementations, it isnot limited thereto. The examples provided herein are illustrative, andother variations and modifications are possible and contemplated. Theforegoing specification is intended to cover all such modifications andvariations.

This application is a Divisional of U.S. patent application Ser. No.13/152,450, filed Jun. 3, 2011, and which claims the benefit of U.S.Provisional Patent Application 61/396,969, filed Jun. 4, 2010, each ofwhich is incorporated herein by reference in its entirety.

1. A method of fabricating a capacitive environment sensor integratedinto a complementary metal oxide semiconductor (CMOS) comprising:selectively etching a CMOS dielectric of a CMOS stack having an exteriorsurface, the CMOS stack comprising, in order of decreasing distance fromthe exterior surface, a first metal layer, a second metal layer and athird metal layer comprising a plurality of gaps, wherein the CMOSdielectric is disposed between the first, second and third metal layersand within the plurality of gaps in the third metal layer, and the firstand third metal layers are electrically connected to the second metallayer by metal vias, the selective etching producing one or morepassages extending from the exterior surface of the semiconductor to thesecond metal layer, through the plurality of gaps in the third metallayer; and selectively etching the CMOS stack to remove the second metallayer, thereby forming a gap between metal vias electrically connectedto the first metal layer and the metal vias electrically connected tothe third metal layer.
 2. The method of claim 1, further comprisingfilling the gap between the metal vias electrically connected to thefirst metal layer and the metal vias electrically connected to the thirdmetal layer with an environment-sensitive dielectric material.
 3. Themethod according to claim 2, wherein the environment-sensitivedielectric material is a polymer.
 4. The method according to claim 2,wherein the environment-sensitive dielectric material is selected fromthe group consisting of: polyimide, polyisobutylene, polydimethylsiloxane, polycarbonate urethane, polyethylene-co-vinylacetate,siloxanefluoro alcohol, polyepichlorohydrine, cyanopropyl methylphenylmethyl silicone, and dicyanoallyl silicone.
 5. The method of claim2, further comprising filling the one or more passages with theenvironment-sensitive dielectric material. 6-9. (canceled)
 10. Themethod according to claim 1, wherein the first second and third metallayers are Al or Cu.
 11. The method according to claim 1, wherein theCMOS dielectric is SiO₂.
 12. The method according to claim 1, whereinthe etching of the second metal layer is accomplished by depositingetchant into at least one of the one or more passages formed in the CMOSdielectric.
 13. The method according to claim 2, wherein the gap betweenthe metal vias electrically connected to the first metal layer and themetal vias electrically connected to the third metal layer is filled byinjecting environment-sensitive dielectric material into at least one ofthe passages formed in the CMOS dielectric.
 14. The method according toclaim 1, wherein CMOS dielectric pillars extend completely through thesecond metal layer, preventing the gap between the metal viaselectrically connected to the first metal layer and the metal viaselectrically connected to the third metal layer from collapsing when thesecond metal layer is removed.
 15. The method according to claim 14,wherein the CMOS dielectric pillars are located about a periphery of atleast one of the one or more passages.
 16. The method according to claim1, further comprising fabricating one or more additional capacitiveenvironment sensors into the CMOS stack. 17-27. (canceled)
 28. A methodof fabricating a capacitive environment sensor integrated into acomplementary metal oxide semiconductor (CMOS) comprising: selectivelyetching a CMOS dielectric of a CMOS stack having an exterior surface,the CMOS stack comprising, in order of decreasing distance from theexterior surface, a first metal layer, a second metal layer and a thirdmetal layer comprising a plurality of gaps, wherein the CMOS dielectricis disposed between the first, second and third metal layers and withinthe plurality of gaps in the third metal layer, the second metal layercomprises a core metal layer of one composition disposed between twometal adhesion layers of another composition and the first and thirdmetal layers are electrically connected to an adjacent metal adhesionlayer of the second metal layer by metal vias, the selective etchingproducing one or more passages extending from the exterior surface ofthe semiconductor to the second metal layer through the plurality ofgaps in the third metal layer; and selectively etching the CMOS stack toremove the core metal layer of the second metal layer, but not the metaladhesion layers, thereby forming a gap between the two metal adhesionlayers.
 29. The method according to claim 28, wherein the metal adhesionlayers are Ti, W, Ta or a combination thereof.
 30. The method of claim28, further comprising filling the gap between the two metal adhesionlayers with an environment-sensitive dielectric material.